Method of fabricating a high-voltage transistor

ABSTRACT

A method for making a high voltage insulated gate field-effect transistor with multiple JFET conduction channels comprises successively implanting a dopant of a first conductivity type in a first substrate of a second conductivity type so as to form a first plurality of buried layers disposed at a different vertical depths. The first substrate is flipped over and then bonded to a second substrate of the first conductivity type. After the first substrate has been thinned, another set of implants are successively performed so as to form a second plurality of buried layers in stacked parallel relationship to the first plurality of buried layers.

This is a Continuation of U.S. application Ser. No. 09/723,957 filed Nov. 27, 2000, now U.S. Pat. No. 6,468,847 which application is assigned to the assignee of the present continuation application.

FIELD OF THE INVENTION

The present invention relates to high voltage field-effect transistors. More specifically, the present invention relates to processes for fabricating high voltage field-effect transistor structures that include a high-voltage junction field-effect transistor.

BACKGROUND OF THE INVENTION

It is conventional to construct a high-voltage, insulated-gate, field-effect transistor (HVFET) having a high breakdown voltage and a low “on-state” resistance. To accomplish this end, practitioners in the art have used an insulated gate field-effect transistor (IGFET) placed in series with a high-voltage junction field-effect transistor (JFET). Such a transistor is capable of switching at high voltages, has low values of on-state resistance, and has insulated-gate control. Moreover, the HVFET may advantageously be fabricated near low voltage logic transistors on a single integrated circuit chip to form what is commonly referred to as a power integrated circuit (PIC).

Lateral HVFETs with a JFET conduction channel have been used in power conversion applications such as in AC/DC converters for offline power supplies. One goal in such devices is to produce a transistor having a high breakdown voltage (V_(bd)) using as small a surface area as possible. In order to achieve high breakdown voltage in these devices is necessary to accurately control the amount of charge in the JFET conduction channel(s) and also in each the JFET gate layers. For this reason, it is desirable to fabricate such devices using a process that minimizes variance in the charge of each layer.

It is also desirable to fabricate HVFETs that occupy as small a surface area as possible to realize a given on-state resistance. The figure of merit often used is known as specific on-resistance (R_(sp)), which is the product of on-state resistance and surface area. A lower R_(sp) allows a smaller HVFET transistor to be used to meet the on-state resistance requirements of a given application, which reduces the area and, respectively, the cost of the PIC. One way of reducing the on resistance of a HVFET is to incorporate multiple JFET conduction channels into the transistor device.

Another goal in the art is to provide a highly manufacturable HVFET design that consistently delivers the required combination of V_(bd) and R_(sp) over a range of normal process variances. To realize this goal, the manufacturing process should introduce minimal variance in the critical device parameters, and the HVFET should exhibit minimal sensitivity to process variations.

To try to achieve the aforementioned goals, researchers and engineers have experimented with a variety of different structures and processing methods. For example, U.S. Pat. Nos. 5,146,298 and 5,313,082 both describe a method of fabricating an HVFET with multiple JFET conduction channels. The '082 patent teaches a HVFET in which two JFET channels are arranged in parallel to increase charge and reduce R_(sp). A triple diffusion process is disclosed, in which three separate implant and diffusion steps are required to form a HVFET (see FIG. 1 of the '082 patent) that includes N-type top layer 28, P-layer 27, and N-type extended drain region 26. The multiple layers of alternating conductivity types is fabricated by implanting, and then diffusing, dopants into the semiconductor substrate. That is, according to the '082 patent, the N-well region, the P-type buried region, and the N-type extended drain region are all diffused from the surface.

One shortcoming of this prior art approach is that each successive layer is required to have a surface concentration that is higher than the preceding layer, in order to fully compensate and change the conductivity type of the corresponding region. Diffusion of dopants from the surface makes it very difficult to maintain adequate charge balance among the layers. In addition, the heavily doped p-n junction between the buried layer and drain diffusion region degrades the V_(bd) of the device. The concentrations also tend to degrade the mobility of free carriers in each layer, thereby compromising the on-resistance of the HVFET. As a result of these difficulties, this method of manufacture is generally limited to producing HVFET devices having no more than two JFET conduction channels.

Another method of fabricating an HVFET with multiple JFET conduction channels is disclosed in U.S. Pat. No. 4,754,310. The '310 patent teaches a method of construction that consists of epitaxially depositing material of alternating conductivity types and then forming V-shaped grooves to contact the resulting plurality of layers. This method suffers, however, from the high costs associated with multiple epitaxial deposition processing steps and the formation of the grooves. Furthermore, it is difficult to precisely control the charge in each layer formed by epitaxially deposition. As noted previously, proper charge control is crucial to achieving a device that is characterized by a consistently high breakdown voltage.

A similar method of fabricating an HVFET with multiple JFET conduction channels is described in an article by Fujihira entitled, “Theory of Semiconductor Superjunction Devices,” Jpn. J. Appl. Phys., Vol. 36, pp. 6254-6262 (October 1997). Fujihira also teaches the technique of epitaxial growth and the formation of grooves to fabricate the HVFET. This method suffers from the same charge control problems and high manufacturing cost discussed above.

Yet another method of fabricating an HVFET with multiple JFET conduction channels is disclosed in U.S. patent application Ser. No. 09/245,029, filed Feb. 5, 1999, of Rumennik, et. al., which application is assigned to the assignee of the present application. Rumennik teaches the use of multiple high-energy implants through the surface of the semiconductor substrate to form a plurality of buried layers. One drawback of this approach, however, is that the number and maximum depth of the buried layers is limited by the available implantation energy. For example, the maximum boron implantation energy available from a typical high-energy implanter is about 7 MeV. Using the techniques disclosed in Rumennik, such an implanter would allow for the formation of four separate buried layers, providing five JFET conduction channels, with a corresponding specific on-resistance of about 6 ohm-mm².

Thus, there still exists a need for a reliable, economically efficient method of fabricating HVFETs with multiple JFET conduction channels that overcomes the problems associated with the prior art.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is illustrated by way of example, and not limitation, in the figures of the accompanying drawings, wherein:

FIGS. 1A-1E are cross-sectional side views illustrating the fabrication of a high-voltage, field-effect transistor (HVFET) device structure in accordance with one embodiment of the present invention.

FIG. 2 is a cross-sectional side view of a HVFET fabricated according to an alternative embodiment of the present invention.

FIG. 3 is a cross-sectional side view of a HVFET fabricated according to another alternative embodiment of the present invention.

FIG. 4 is a cross-sectional side view of a HVFET fabricated according to yet another alternative embodiment of the present invention.

FIG. 5 is a cross-sectional side view of a HVFET fabricated according to still another alternative embodiment of the present invention.

FIG. 6 is a cross-sectional side view of a HVFET fabricated according to a further alternative embodiment of the present invention.

DETAILED DESCRIPTION

In the following description, numerous specific details are set forth, such as material types, structures, particular processing steps, etc., in order to provide a thorough understanding of the present invention. Practitioners having ordinary skill in the semiconductor arts will understand that the invention may be practiced without many of these details. In other instances, well-known elements, techniques, and processing steps have not been described in detail to avoid obscuring the invention.

The present invention relates to a method for fabricating a high-voltage field-effect transistor with multiple JFET conduction channels that provide a low on-state resistance for a given breakdown voltage. While n-channel HVFETs are presented herein for illustrative purposes, p-channel HVFETs can also be fabricated utilizing complementary processing techniques that appropriately reverse the conductivity types associated with the various regions and layers.

Referring now to FIG. 1A, a cross-sectional view of an N-type substrate 10 following formation of the buried regions 15-17 is shown. Substrate 10 comprises a top surface 18 and a bottom surface 19. In the described embodiment, substrate 10 is uniformly doped N-type. The doping level is chosen to provide the required amount of charge in each of the JFET conduction channels of the final HVFET device. The JFET conduction channels comprise the N-type regions disposed between the P-type buried layers 15 in FIG. 1A. In accordance with the present invention, buried layers 15 and their associated JFET conduction channels are formed in a laterally extended region of the substrate that eventually will be part of the laterally extended drain of the completed HVFET.

Practitioners in the art will appreciate that the laterally extended region may also comprise the high-voltage portion of another lateral power device. For example, high-power diodes, JFETS, LIGBTs, and so on may also be incorporated in the laterally extended region of the HVFET.

In one embodiment, substrate 10 includes an etch-stop layer and/or cleave plane to facilitate control of the final thickness of this layer (after wafer bonding and etch-back, as will be described shortly below). It is appreciated that substrate 10 may comprise an epitaxial layer. In another embodiment, substrate region 10 may comprise an implanted and diffused N-well region formed in a uniformly doped P-type substrate material.

The buried regions 15-17 of FIG. 1A are formed using conventional photolithography followed by implantation of a P-type dopant such as boron into substrate 10. For example, masking layer members 11, 12, and 13 are formed on top surface 18 and multiple implantation steps have been performed through top surface 18 (represented by arrows 14) to form the multiple P-type buried layers shown. The dose and energy for each of the ion implants is chosen to provide the required amount of charge in each of the buried layers 15, and also in the corresponding JFET conduction channels. The N-type conduction channels around the P-type buried layers 15 provide paths for current to flow in the extended drain region of the HVFET.

For a given implantation energy, the thickness of masking layer members 11-13 affects the penetration of the dopant into substrate 10. As can be seen, both masking members 11 and 12 have a thickness t₁ that effectively prevents any dopant ions from penetrating into the substrate material. Conversely, where top surface 18 is exposed, the depth of individual buried layer regions 15 and 16 is determined according to the energy and dose of each implant step.

Note that according to the embodiment shown, corresponding P-type buried layer regions 15 and 16 (labeled PB′) are formed simultaneously using the same implantation step. For instance, buried layer regions 15 a & 16 a are formed at the same depth in substrate 10 utilizing the same implantation step. Similarly, buried layer region pairs 15 b & 16 b, 15 c & 16 c, and 15 d & 16 d are each formed using the identical implant processing step. The difference in vertical depth between the buried layer regions 15 a-d and 16 a-d relative to the top surface is due to differences in implant energy and dose associated with each of the multiple implantation steps. The exposed portions of surface 18 may also be covered with additional layers of material, such as oxide, to screen or block the implants.

It should be understood that the PB′ buried layer regions 16 are optionally included in the process of the present invention. In other words, alternative embodiments may exclude these PB′ buried layer regions. In the embodiment represented by FIGS. 1A-1E, buried layer regions 16 are formed an area beneath the substrate surface where the source region of the HVFET will be located.

FIG. 1A also illustrates how a difference in thickness of the surface masking layer may be utilized to achieve buried layer regions 17 disposed at different depths within substrate 10 relative to corresponding regions 15 and 16. In this case, masking layer member 13 is purposefully formed to have a thickness t₂ that is less than thickness t₁. Thickness t₂ is chosen to cause buried layer regions 17 a-17 d (labeled PB″) to be formed at a slightly shallower, offset depth as compared to corresponding buried layer regions 16 a-16 d. Masking layer members 11-13 may comprise oxide, nitride, photoresist, or any other suitable material.

By appropriate selection of the thickness t₂ of masking member 13 and implant energy, the adjacently formed PB′ and PB″ buried layer regions will form a continuous region of P-type doping extending below surface 18 of substrate 10. This continuous P-type region may be used to isolate the HVFET from other circuitry fabricated in the same substrate. This aspect of the invention will be discussed in more detail below.

It should be understood that for this embodiment the PB″ buried layer regions 17 a-17 d are formed using the same implantation steps used to form the corresponding PB′ buried layer regions 16 a-16 d (and also P-type buried layer regions 15 a-15 d). Thus, no additional processing steps are required to form the optional PB′ and PB″ buried layer regions.

FIG. 1B is a cross-sectional view of substrate 10 after it has been flipped over and bonded to P-type substrate 20. The bonding of top surface 18 to substrate 20 may be achieved by conventional wafer bonding techniques. During the wafer bonding process, a relatively low temperature is preferably maintained to avoid out-diffusion of the P-type buried layers 15-17. Since the bonded interface between substrate 10 and substrate 20 will lie within the depletion region of the HVFET under reverse bias, the interface should ideally be free of traps, voids, and contamination.

FIG. 1B also shows the device at a stage in the fabrication process after thinning of substrate 10. According to the embodiment of FIG. 1B, thinning occurs from the top down, i.e., from surface 19 down toward surface 18. After thinning, the new top surface of the device substrate is denoted by numeral 19′ in FIG. 1B. Thinning may be performed using any one of a variety of techniques, including conventional chemical etching, mechanical, or chemo-mechanical methods.

In one embodiment, an etch stop layer or cleave plane is embedded within substrate 10 where surface 19′ is to be located, so as to provide good control over the final thickness of substrate 10. As will be seen shortly, the final thickness of substrate 10 is important in establishing the spacing between buried layer region 15 d and buried layer region 15 e, as formed in subsequent processing steps.

FIG. 1C is a cross-sectional view of the device of FIG. 1B following formation of uppermost buried layer regions 15 e-15 h, 16 e-16 h, and 17 e-17 h. (Once again, it should be understood that buried layers 16 and 17 are optionally included in the embodiment shown.) Uppermost buried regions 15 e-15 h, 16 e-16 h, and 17 e-17 h may be formed utilizing similar photolithography and ion implantation steps used to form the underlying buried layer regions 15 a-15 d, 16 a-16 d, and 17 a-17 d. In this example, an optional oxide layer 30 is first grown or deposited on surface 19′ of substrate 10. Oxide layer 30 may become the field oxide of the final HVFET. Ideally, optional oxide layer 30 is produced using low temperature processing techniques to minimize out-diffusion of the previously formed buried layers. Another layer 32 may also be provided with the same thickness as layer 30.

Masking layer members 31, 34 and 35 are formed over surface 19′ and multiple implantation steps are performed (represented by arrows 37) to form the multiple, uppermost P-type buried layers and corresponding JFET conduction channels shown. The dose and energy of each implant is chosen to provide the required amount of charge in each of the uppermost P-type buried layers and also in the corresponding JFET conduction channels. The thickness of members 34 and 35 (t₁) is chosen to completely block the implant.

The thickness of layer 30 allows implant ions to penetrate to a certain depth in the substrate. Because buried layer regions 15 e-15 h and 16 d-16 h are each formed using the same implantations, the relatively thinner masking layer 31 (t₃) produces a set of buried layer regions 16 d-16 h that are disposed more deeply within substrate 10, as compared with the corresponding buried layer regions 15 d-15 h, respectively. As shown, the relatively thinner masking layer 31 produces a buried layer region 16 e that is merged with underlying buried layer region 16 d. In this embodiment, merging of regions 16 d and 16 e is important to establish a continuous P-type isolation region that extends from the source region down to substrate 20 in the completed device structure. This continuous P-type isolation region may be used to isolate the HVFET from other circuitry.

The cross-sectional side view of FIG. 1C also shows the optional inclusion of P-type buried layer regions 17 e-17 h (labeled PB″) formed adjacent, and connected to, corresponding PB′ buried layer regions 16 e-16 h. These PB″ buried layer regions are formed by the same implantation steps used to form the PB′ buried layer regions, so that no extra processing steps are required. In the example shown, a masking layer 32 having a thickness greater than that of layer 31 is formed on surface 19′ prior to the implantation steps. The greater thickness of masking layer 32 relative to layer 31 causes the respective buried layer regions 17 e-17 h to be formed at relatively shallower depths as compared to their counterpart buried layer regions 16 e-16 h. Here again, appropriate selection of mask layer thickness' and implantation energies causes the PB′ and PB″ buried layer regions to merge, thereby forming a continuous region of P-type doping that extends from just beneath surface 19′ down to substrate 20.

Once the extended drain region or drift region of the HVFET has been formed according to the preceding steps, the following fabrication steps may be utilized to complete the device.

FIG. 1D illustrates a cross-sectional view of an insulated-gate HVFET after the formation of source region 42, drain region 47, and polysilicon gate 43, which is insulated from substrate 10 by a thin gate oxide layer 44. These structural features may be formed by conventional processing techniques.

In the method of manufacturing a high-voltage IGFET, the growth or deposition of gate oxide layer 44 follows the high-energy implantation steps that form the buried layers. After formation of gate oxide layer 44, polysilicon gate 43 may be deposited and patterned. In the particular embodiment shown, a P-type body region 40 is formed prior to the N-type implantation used to simultaneously form source and drain regions 42 and 47, respectively. For clarity purposes, body region 40 is shown merged with P-type buried layer regions 17 g, 16 h, and 17 h. Region 40 may be formed using conventional angled implantation techniques.

The embodiment of FIG. 1D also shows the optional formation of P-type region 41 to increase the integrity of the source-to-substrate connection and reduce susceptibility of the device to parasitic bipolar effects.

Another optional processing step shown in FIG. 1D is the formation of a drain polysilicon member 45. Drain polysilicon member 45 is shown insulated from substrate 10 by oxide layer 46 and extending over a portion of field oxide 30. Similarly, a portion of polysilicon gate 43 is made to extend over part of field oxide 30. These polysilicon extensions are useful for field-plating purposes. Oxide layers 44 and 46 may be formed simultaneously by the same process steps, as may polysilicon layers 43 and 45.

FIG. 1E is a cross-sectional side view of the embodiment of FIG. 1D following the formation of an inter-level dielectric layer 50, etching of contact openings, and deposition and patterning of a conductive layer. The inter-level dielectric layer 50 may be deposited (and then densified or reflowed, if necessary). By way of example, dielectric layer 50 may comprise a low-temperature oxide (LTO).

Conventional photolithography and etching steps are employed to form contacts to the source and drain regions. A suitable conductive material such as aluminum, titanium alloy, or copper is commonly deposited, patterned, and etched to form respective source and drain electrodes 52 and 51, which provide electrical contact to source and drain regions 42 and 47, respectively. Note that electrode 52 also contacts region 41 to provide electrical connection to region 40 and substrate 20 (via layers 16 and 17) in the embodiment shown. Practitioners in the art will appreciate that the HVFET of the illustrated embodiment is typically operated with the source and substrate connected to ground, which provides the double-sided JFET with enhanced switching characteristics.

Furthermore, each of electrodes 51 and 52 may include a portion that extends over dielectric layer 50 to act as a field plate member. These field plate members reduce peaks in the localized electric field, thereby increasing the breakdown voltage of the transistor.

In the completely fabricated device, the IGFET channel region comprises the area of body region 40 directly under gate 43 between N+ source region 42 and the extended drain region, which begins at the lateral boundary of body region 40 and substrate 10. Also note that the embodiment of FIG. 1E shows each of the P-type buried layer regions 15 a-15 h is surrounded above, below and laterally by N-type material of substrate 10; the JFET conduction channels being formed in a parallel manner between the corresponding buried layer regions. It is appreciated that the separation of buried layer regions 15 from N+ drain region 47 by a portion of the N-type substrate 10 improves the breakdown voltage of the transistor.

When the HVFET is in the on-state, electron current flows from the source diffusion region 42 through the IGFET channel region beneath gate 43, then through the parallel JFET channels formed between buried layer regions 15 a-15 h, and finally to drain diffusion region 47. The formation of a large number of parallel-configured P-type buried layers (e.g., 8 in this example) and their corresponding JFET conduction channels (e.g., 9 in this example) greatly reduces the resistance of the extended drain region as compared to a conventional device.

In one particular implementation of the HVFET process, each of the buried layer regions 15 includes a connection to substrate 20 (not shown in the cross-sectional views 1A-1E) or another region having substantially the same potential. This insures that buried layer regions 15 are not left floating (electrically).

Another embodiment of the invention is shown in FIG. 2. This embodiment obviates the need for a uniformly doped N-type starting substrate material. Instead, a diffused N-well region 60 is initially formed in a P-type substrate 68 using ordinary processing techniques. N-well region 60 is fabricated prior to introduction of the buried layers 15. In this embodiment, the HVFET can be effectively isolated from other circuitry by the lateral junction of N-well 60 and P-type substrates 20 and 68. Thus, the approach of FIG. 2 obviates the need for the aforementioned steps associated with the formation and merging of the PB′ and PB″ layers.

Yet another alternative embodiment is shown in the cross-sectional side view of FIG. 3. Rather than forming a continuous P-type region by merging PB′ and PB″ layers 16 and 17, a deep P-type diffusion into the N-type substrate 10 is performed prior to formation of the first plurality of buried layers (e.g., regions 15 a-15 d). During the subsequent formation of P-type body region 40, deep P+ diffusion region 61 merges with region 40 to provide isolation of the HVFET from other circuitry fabricated on the same substrate.

FIG. 4 shows still another embodiment of the invention in which a trench isolation oxide region 62 is formed in substrate 10 to electrically isolate the HVFET. By way of example, the trench may be formed by conventional etching methods after substrate 10 has been bonded to substrate 20 and thinned, as described previously. A low-temperature oxide may be used to fill the isolation trench region.

The alternative embodiments shown in FIGS. 5 & 6 involve replacing the gate and/or drain regions of the device with trench structures to provide more uniform current flow through the parallel-configured JFET conduction channels.

For example, the device structure shown in the cross-section of FIG. 5 may be produced by replacing the formation of gate 43 (see FIG. 1E) with the following processing steps. First, a deep trench region is etched down to substrate 20 in substrate 10 adjacent to the source region of the device. Next, gate oxide layer 64 is formed followed by formation of a vertical polysilicon gate 63. The remainder of the HVFET may be completed in accordance with the teachings associated with FIGS. 1D & 1E.

The device structure shown in the cross-section of FIG. 6 may be produced by replacing the formation of N+ drain diffusion region 47 with etching and diffusion steps that form a N+ drain trench region 67 extending deeply into substrate 10. In this particular embodiment, the drain electrode includes a segment 71 that extends into the trench to contact drain trench region 67.

Although the processing steps in the foregoing description are for fabrication of a n-channel HVFET, it is appreciated that a p-channel HVFET can be realized by simple reversal of the conductivity types employed to form the various regions/layers.

It should be understood that although the present invention has been described in conjunction with specific embodiments, numerous modifications and alterations are well within the scope of the present invention. Accordingly, the specification and drawings are to be regarded in an illustrative rather than a restrictive sense. 

I claim:
 1. A method of fabricating a high-voltage field-effect transistor (HVFET) comprising: forming a first plurality of buried layers of a first conductivity type in a first substrate of a second conductivity type, the first substrate having first and second opposing surfaces, each of the first plurality of buried layers being disposed it a different vertical depth in the first substrate relative to the first surface such that a corresponding first plurality of JFET conduction channels of the second conductivity type are formed within the first substrate; bonding the first surface of the first substrate to a second substrate of the first conductivity type; thinning the first substrate from the second surface; forming a second plurality of buried layers of the first conductivity type in the first substrate, each of the second plurality of buried layers being disposed at a different vertical depth in the first substrate relative to the second surface such that a corresponding second plurality of JFET conduction channels of the second conductivity type are formed within the first substrate.
 2. The method according to claim 1 wherein each of the first plurality of buried layers is spaced apart from the first surface.
 3. The method according to claim 1 wherein each of the second plurality of buried layers is spaced apart from the second surface.
 4. The method according to claim 1 wherein thinning is performed by etching back the second surface to an etch slop layer or cleave plane embedded within the first substrate.
 5. The method according to claim 1 wherein the first and second JFET conduction channels are formed in a parallel configuration.
 6. The method according to claim 1 wherein the first conductivity type is P-type and the second conductivity type is N-type.
 7. The method according to claim 1 wherein the second plurality of buried layers are spaced vertically apart from the first plurality of buried layers.
 8. A method of fabricating a high-voltage field-effect transistor (HVFET) comprising: forming a well region of a first conductivity type in a first substrate ate second conductivity type, the first substrate having first and second opposing surfaces, the well region having a laterally extended portion with a lateral boundary; forming a first plurality of buried layers of a first conductivity type in a first substrate of a second conductivity type, the first substrate having first and second opposing surfaces, each of the first plurality of buried layers being disposed at a different vertical depth in the first substrate relative to the rust surface such that a corresponding first plurality of JFET conduction channels of the second conductivity type are formed within the first substrate; bonding the first surface of the first substrate to a second substrate of the second conductivity type; thinning the first substrate from the second surface; forming a second plurality of buried layers of the first conductivity type in the first substrate, each of the second plurality of buried layers being disposed at a different vertical depth in the first substrate relative to the second surface such that a corresponding second plurality of JFET conduction channels of the second conductivity type are formed within the first substrate.
 9. The method according to claim 8 wherein each of the first plurality of buried layers is spaced apart from the first surface.
 10. The method according to claim 8 wherein each of the second plurality of buried layers is spaced apart from the second surface.
 11. The method according to claim 8 wherein thinning is performed by etching back the second surface to an etch stop layer or cleave plane embedded within the first substrate.
 12. The method according to claim 8 wherein the first and second JFET conduction channels are formed in a parallel configuration.
 13. The method according to claim 8 wherein the first conductivity type is P-type and the second conductivity type is N-type.
 14. The method according to claim 8 wherein the second plurality of buried layers are spaced vertically apart from the first plurality of buried layers. 